1. Field of the Invention
The present invention relates to an ultrasonic motor driving device which controls the drive frequency for the ultrasonic motor in accordance with phase difference between drive voltage and monitor voltage.
2. Related Background Art
There have been known ultrasonic motor driving devices which control drive frequencies based on phase differences between waveforms of drive voltage applied to a motor drive electrode and waveform of monitor voltage generated at a motor monitor electrode which generates frequency voltage in accordance with vibrations of an elastic member of the motor. The contol is such as to achieve a predetermined phase difference which provides stable operation of the motor. See, for example, Japanenes Patent Laid-Open No. 61-251490.
Such a conventional driving device will be described with reference to FIG. 4.
Drive voltage applied to a drive electrode 100a of an ultrasonic motor 100 is fed back to a waveform shaper 1, by which it is converted to square-wave voltage of a predetermined level. On the other hand, monitor voltage generated at a monitor electrode 100c is fed back to another waveform shaper 2, by which it is converted to square-wave voltage of a predetermined level. These square-wave voltages are inputted to a phase difference detector 3 illustrated in detail in FIG. 5.
As shown in FIG. 5, the phase difference detector 3 is composed of AND gates 31, 32, an SR flip flop circuit 33, a buffer 34, a resistor 35 and a capacitor 36. The above-mentioned square-wave drive voltage is sent to one of two input terminals of one of the AND gates 31. The square-wave monitor voltage is sent to one of two terminals of the other AND gate 32. Then, an output terminal Q of the flip flop circuit 33 outputs pulse voltage having a pulse width in accordance with a phase difference between the square-wave voltages inputted to the AND gates 31 and 32, i.e. a phase difference .PHI. between the drive voltage and the monitor voltage. The pulse voltage is converted to a voltage level in accordance with a phase difference .PHI., as shown in FIG. 6, by an integration circuit composed of the buffer 34, the resistor 35 and the capacitor 36.
An output of the phase difference detector 3 is sent to a terminal (-) of an error amplifier 42 in a drive frequency setting unit 4. Another terminal (+) of the error amplifier 42 receives a voltage in accordance with a reference phase difference .PHI.K set by a reference phase difference setter 41. The error amplifier 42 compares the phase difference .PHI. from the phase difference detector 3 with the reference phase difference .PHI.K, and then amplifies a voltage in accordance with a difference between the phase differences (.PHI.K-.PHI.). The voltage is sent to a loop filter circuit composed of a resistor 43 and a capacitor 44, and then to a voltage controlling oscillator 45 (referred to as a "VCO" hereinafter). The VCO 45 generates a frequency signal having a frequency in accordance with an inputted voltage level and then sends it to a phase shifter 5.
The phase shifter 5 converts the frequency signal to a pair of frequency signals having a phase difference of .pi./2 therebetween and outputs them to a power amplifier 6. When a start input signal is at Hi-level, the power amplifier 6 amplifies the power of these frequency signals and applies the amplified signals to the drive electrodes 100a and another drive electrode 100b of the ultrasonic motor 100.
Thus, the conventional driving device performs a feedback control by which the phase difference .PHI. between a monitor voltage and a drive voltage of a current drive state become equal to the reference phase difference .PHI.K so that drive frequency of the ultrasonic motor 100 becomes equal to a frequency FK corresponding to the reference phase difference .PHI.K.
FIG. 7 shows variations of a drive speed N and phase difference .PHI. with relation to a drive frequency F of an ultrasonic motor. In the figure, a frequency range FD is the range of drive frequency usually used for ultrasonic motors. A frequency FR is the resonance frequency which causes the largest vibrations of the elastic member of the ultrasonic motor. It is known that drive frequencies very close to the resonance frequency FR cause the ultrasonic motor to operate unstably. There are other frequencies with which the elastic member resonates; one frequency is lower than and another is higher than the resonance frequency FR, as shown in FIG. 7. Although ultrasonic motors can be driven by frequencies in ranges slightly higher than these other resonance frequencies, such frequency ranges are usually not used because if they are used, the motor operates unstably and rotates in a reversed direction.
While the above-described conventional device controls the drive frequency so that a phase difference .PHI. becomes equal to the reference phase difference .PHI.K, if a current drive frequency is slightly lower than the drive frequency FK, i.e. a current phase difference .PHI. is smaller than the reference phase difference .PHI.K (.PHI.K-.PHI.&gt;0), the error amplifier 42 accordingly increases its output voltage and thus the capacitor 44 is charged. As the terminal voltage of the capacitor 44 increases, the frequency of a frequency signal from the VCO 45 increases. When the frequency of the drive voltage applied to the ultrasonic motor 100 reaches the drive frequency FK, i.e. a current phase difference .PHI.= the reference phase difference .PHI.K, the error amplifier 42 stops charging the capacitor 44. Thus, the capacitor 44 maintains its terminal voltage at such a level that the frequency outputted by the VCO 45 is equal to the drive frequency FK. In this way, the ultrasonic motor 100 is driven by the frequency FK.
If a current drive frequency is slightly higher than the frequency FK, i.e. a phase difference .PHI.&gt;.PHI.K, the error amplifier 42 accordingly reduces its output, so that the capacitor 44 discharges to reduce its terminal voltage. In this manner, frequency outputted by the VCO 45 decreases to the frequency FK, which makes a phase difference .PHI. equal to the reference phase difference .PHI.K. Thus, the ultrasonic motor 100 is driven by the drive frequency FK.
While an ultrasonic motor is being operated, a detected phase difference .PHI. sometimes varies because of a sudden load change, noise interference in a controlling circuit, particularly in a monitor voltage waveform shaper, etc. Also, when an ultrasonic motor is started, the ultrasonic motor operates unstably for a moment, so that the detected phase difference .PHI. varies.
In a conventional driving device as described above, an output of an error amplifier substantially changes and thus a frequency outputted by the VCO substantially changes as the phase difference .PHI. varies during operation or start-up of an ultrasonic motor. As a result, a feedback control system converges a drive frequency to a frequency which is outside the usual drive frequency range FD but which provides a phase difference equal to the reference phase difference .PHI.K, so the ultrasonic motor operates unstably. As shown in FIG. 7, since phase difference .PHI. goes up and down as the drive frequency F is continuously varied, the motor operation driven by frequencies other than the drive frequency FK, such as FL2, FL1, FH1 and FH2, provide a phase difference equal to the reference phase difference .PHI.K.
If output frequency of the VCO 45 substantially decreases, with a change in phase difference .PHI., to a level lower than the frequency FL1, the error amplifier 42 operates to decrease, not increase, the output frequency of the VCO 45 since a current phase difference .PHI. is greater than the reference phase difference .PHI.K (.PHI.K-.PHI.&lt;0), as described above. Thus, a current drive frequency F further decreases until it reaches the frequency FL2, which provides a phase difference .PHI.= the reference phase difference .PHI.K. In this manner, the frequency control converges to the frequency FL2. Driven by the frequency FL2, the ultrasonic motor 100 operates unstably, as described above.
On the contrary, if the output frequency of the VCO 45 substantially increases, with a change in phase difference .PHI., to a level higher than the frequency FH1, the error amplifier 42 operates to increase, not decrease, the output frequency of the VCO 45 since a current phase difference .PHI. is smaller than the reference phase difference .PHI.K (.PHI.K-.PHI.&gt;0). Thus, a current drive frequency F further increases to reach the frequency FH2, which provides a phase difference .PHI.= the reference phase difference .PHI.K. In this manner, the frequency control converges to the frequency FH2. Driven by the frequency FH2, the ultrasonic motor 100 operates unstably, as described above.
There can be considered methods to avoid such a disadvantage of the conventional devices, e.g. reducing the amplification ratio of the error amplifier 42 so that changes in the frequency outputted by the VCO 45 will be suppressed, or increasing the time constant of the loop filter composed of the resistor 43 and the capacitor 44 so that the responsiveness of the feedback control system will be reduced. However, such methods can not be employed since control precision and control responsiveness decrease if any of such methods are used.